India and China are due to reopen their land border this week, for the first time since an intense frontier war more than 40 years ago. This political breakthrough comes as both countries race to become the economic giants of the world in the next few decades.

BBC diplomatic correspondent James Robbins has been travelling in both India and China to assess the scale of the challenge they face and the impact their rise will have on all of our lives.

He starts a series of special reports for the BBC News website with exclusive access to the border in India's Sikkim state, still a restricted military zone.

Above the clouds, we followed a trail on the old Silk Road high into the Himalayas. We were in India's Sikkim state heading towards China on a road which, until now, led only to a dead end.

At 14,500 feet (over 4,400 metres) we reached the Indian side of the border with China.

It has been sealed since 1962, when there was a fierce border war here, with Indian and Chinese infantry backed by heavy artillery fighting each other and against altitude sickness over each disputed ridge.

Now the entire ridge is fortified. It looks like another Wall of China - a small wall, rather than a great one, but fearsome nonetheless.

After weeks of tough negotiations with the Indian authorities, we were the first foreign TV team to be allowed up here as a "cold war" across the Himalayas draws to a close.

Mountain mail

Colonel Chauhan of the 5th Battalion of India's Grenadier Guards gave us a breakfast of porridge and cucumber sandwiches before we climbed the final few hundred feet (the final hundred metres) to the top of the Nathu La Pass.

Exchanging post has been the sides' only human contact for 40 years

Nothing really prepared us for the very human story we would find up there.

Between the Indian and Chinese fortifications is strung a simple barbed wire fence, more a marker than a real barrier now, but - make no mistake - no-one is allowed to cross.

Except, that is, Ye Ling, China's postman, and his Indian counterpart, Mr Thaman.

On a Sunday, we watched the Chinese postman as he was allowed through the wire to the Indian side on his once a week crossing.

On Thursdays it's India's turn to cross in the other direction.

Theirs is the only human contact which has been allowed for more than 40 years.

China's postman himself has been crossing through the wire for 17 of those years.

Initially it's going to be largely symbolic

Col ChauhanIndian army

He is escorted by a soldier of India's 5th Grenadiers - part of a force of thousands strung out along this border.

Inside the border post mail is carefully unpacked under military scrutiny, ready for a bureaucratic ritual with India's postman.

First they weigh and then sign for the precious letters, which are all that is traded across this remotest of borders.

"I ride up here on a horse," Ye Ling told me.

"But in winter when there's really thick snow, I have to clamber up by myself."

Bridging the divide

Now the postman's life is about to be made far easier - political breakthrough means a new Silk Road is being driven through this ridge.

An ancient route is being revived with modern links

Hacked out by hand a new world-changing relationship is being built, as Indian stonebreakers edge ever closer to the Chinese road gangs in yellow hard hats, just over the line dividing the two nations.

This back-breaking work is aimed at one thing:

On Thursday, the two countries intend to open the border for trade.

It's just part of an explosion of business which means billions of dollars flowing to India and to China. It is the big story of our times.

Col Chauhan, commanding officer of Indian troops up here, tells me: "Basically to start with it's going to be a border trade.

"Chinese yak skins in exchange for tea and some local Sikkim liquor. Initially it's going to be largely symbolic, but later on the trade will start building up to a larger scale."

So the two armies, India's and China's, are no longer disputing this border.

Instead, their governments are both pushing for huge shares of the world's trade and wealth.

Together they are determined to build two giant economies in the world that will change all our lives.